The present invention relates generally to magnetic resonance imaging (MRI) and in particular to local coils for use in MRI.
In magnetic resonance imaging, a uniform magnetic field is applied to an imaged object, for example, a patient and a radio frequency (RF) excitation signal pulse is applied to excite nuclei within the patient into resonance. This RF excitation pulse will also referred to herein as a “transmit” signal.
After application of the transmit signal, one or more magnetic gradient fields are superimposed on the uniform magnetic field to spatially encode the precessing nuclei, and nuclear magnetic resonance (NMR) signal from the nuclei are received and processed mathematically to produce an image. The NMR signal will also be referred to herein as a “receive” signal.
The transmit and receive signals may be transmitted and received by loop antennas termed “coils”, such as a whole body coil built into the MRI machine to encompass the entire patient.
Improvement in the signal-to-noise ratio of the receive signal can be obtained by placing “local coils” on the patient. The local coil has a smaller reception pattern than a whole body coil and therefore can focus on a smaller region of interest to obtain a stronger signal and to receive less noise. Locals coils may also provide improved application of the transmit signal.
A common local coil design is the “birdcage” coil which provides two conductive rings separated along a common axis to define the two bases of a cylindrical volume. A number of conductive axial struts are spaced regularly about the circumference of rings to join the rings. The rings may be excited into resonance at the transmit signal frequency so that a traveling wave progressively promotes current flow in each of the struts to produce a highly uniform rotating magnetic vector within the cylindrical volume. The same coil can be used to collect the receive signal, the rings serving to combine the signals from each of the struts inducted by the rotating magnetic vector of the nuclei.
Phased array local coils are multiple loop local coils where the outputs from each loop are independent and may be processed independently to improve signal-to-noise ratio or to obtain additional spatial information. The loops of a phased array coil may be arranged about a cylindrical volume, and thus may resemble and provide similar coverage to a birdcage coil, but unlike the birdcage coil, the phased array coil provides multiple independent signals to the MRI machine in contrast to the birdcage coil which provides a signal as a combination of the current flows in each strut.
In order to obtain independent electrical signals from each loop, a phased array coil must normally provide decoupling between the loops. This decoupling may employ a controlled overlap between adjacent loops or capacitive decoupling networks or the like.
While phased array coils provide advantages for the reception of the receive signals (receive mode), a standard birdcage structure can provide a more uniform field for transmitting the transmit signal (transmit mode). Accordingly, there is considerable interest in creating a hybrid local coil that may operate as a birdcage coil while transmitting and, as a phased array coil when receiving.
Another approach is to combine the physical structures of the phased array coil and birdcage coil in two concentric arrays. One array is activated during transmission and the other during reception. This approach increases the weight and size of the coil and interaction between the two arrays can interfere with the uniformity of the field and/or reduce the signal-to-noise ratio of the received signals.
An alternative approach connects each of the loops of the phased array coil directly with the output to a different phase shifter providing phase shifted transmit signals mimicking the phase shifting provided by the birdcage coil end rings. Unfortunately, variations in the loading of each loop often result in widely differing loop currents creating transmit field distortions that result in poor performance for phase sensitive MRI techniques such as fat saturation sequences.
Variations in the current flows from each phase shifter can be controlled by the addition of a radio frequency amplifier between the phase shifter and each loop. This approach is expensive and significantly increases the bulk and weight of the coil and for this reason may be practical only for whole body coils.